This invention relates to rotory disconnect devices for disconnecting a rotation device once a predetermined rotational speed had been reached. In particular, the invention relates to engine starters having a coupling which advances into engagement with an engine by means of a helical spline arrangement. The invention has particular application to gas turbine engine starters such as air turbine starters used to start gas turbine engines.
The present invention has particular application to a starter coupling which advances into engagement with an engine by means of a helical spline arrangement. Such helical spline arrangements are well known in the art and are described in U.S. Pat. No. 1,144,097, to Vincent Bendix. Briefly, when starter begins to turn, a pinion gear moves longitudinally along helical splines on a starter shaft. When the engine is started, the starter is disengaged and caused to slow down, thereby allowing the pinion to return to its initial position on the starter shaft away from engagement with engine's driven gear.
In a typical application, gas air turbine starters are used on a multi-engine aircraft. Compressed air is provided to each of the aircraft's main engine starters, initally from an auxiliary power unit (APU) and then as bleed air from those main engines which have started. The pilot initiates his start procedure by powering up the APU, which typically uses an electric starter. He would then start number 2 engine, assuming that engine is closest to the APU. At a specified engine speed, the engine is ignited. At a higher speed, but below idle, the starter is disengaged while the engine is continued to be powered up to an idle speed. After number 2 has started, the would then start number 3, followed by numbers 1 and 4 after the number 3 starter is disengaged.
In starting an engine, the start speed is usually less the normal operating speed. It therefore is desirable to separate the starting motor from the main engine once the main engine has started. This is necessary to reduce power loss from the main engine, to prevent the starter motor from wearing due to extended periods of rotation at speeds above that for which the starter motor was designed and to reduce the wear due to ratcheting on the driving and driven jaws of those starter mechanisms equipped with overrunning clutches.
In the case of starters for a gas turbine engines, the engagement of the starter is by means of an overrunning clutch having mating jaws with axially extending, circumferentially-spaced teeth. The teeth on the mating jaws have ramped profiles so as to transmit torque in a driving direction but inhibit the transmission of torque in the reverse direction. This arrangement is shown in U.S. Pat. No. 3,458,091 to W. Fant, et al.
Since the spaced-apart relationship of the starter jaws is necessary when the starter motor is turned off after the main engine has been started, one logical approach is to maintain the spaced-apart relationship at all times when the starter motor is turned off.
When this type of mechanism is used in certain applications, particularly with propeller driven aircraft, vibration at some frequencies causes the starter's driving jaw to advance after engine start. When the starter is inactive, that jaw then comes into contact with the rotating engine half of the coupling (the driven jaw). In the case of air turbine starters, the untimely advance of the starter jaw may also be caused by air which may leak past the starter control valve, thereby causing the starter to improperly motor after the engine has been started. Regardless of the cause, untimely advance of the starter jaw can cause rapid wear and damage.
The aforementioned U.S. Pat. No. 3,458,019 discloses a centrifugal locking device which pivots against the splines in order to latch behind the splines after the starter has become engaged. This permits the driven jaw to be held in a favorable position as long as the locking device is latching against spline teeth.
It is therefore an object of the present invention to provide an improved lock-out arrangement for engine starter engagement couplings.